Tannin staining, an undesirable process which results in aesthetic degradation and loss of decorative value of protective coatings is a problem frequently encountered, for example, with white coatings applied on wood substrates. It is observed usually as yellow-brown coloration or as randomly distributed brown colored spots on freshly applied aqueous white coatings and more particularly, on white coated wood substrates exposed to high, typically condensing, humidity conditions.
Water soluble tannins or tannic acids, natural compounds of complex and non-uniform composition, are the staining species involved, which are abundantly present, especially in redwood substrates.
A significant example of such materials is the group known as hydrolyzable tannins which are esters of hexozes (normally glucose) formed with benzoic acid or its derivatives in variable mole ratios. Their complex chemical composition and structure is consistent with their intricate chemical behavior and physical properties, some of which are relevant to the tannin staining process, i.e.: solubility in water and polar organic solvents, tendency to darken in the presence of air (more specifically in alkaline media) and to form soluble or insoluble, usually dark colored combinations with various metal cations. Similarly, the formation of colored tannate species are observed in the presence of insoluble or partially soluble, various (pigment grade) mineral products, in which case, apparently, the anionic species involved interfere as well with the related process. Notably, tannins' mild acidic character is also well known.
A complex phenomenon, tannin staining includes several concurrent processes: water or vapor penetration of wood substrates, solubilization, diffusion into the coating and darkening of the deposited air-exposed tannin species, among others. It is significant to observe in this sense that the rate of staining is diffusionally controlled and its extent is significantly dependent on the substrate's tannin concentration.
By definition, staining inhibition in the above specified sense implies such capacities of the specialized coating systems, as to interact with dissolved tannin species and to interfere with related diffusion processes, thus causing "in situ" immobilization of the formers and resulting in overall obstruction of the staining process.
There are specialized pigment grade products known by the prior art as "blockers of tannin" or "stain inhibitors", which as functional components of water or solvent based paint formulations, provide such protective capacity to white coatings systems applied on wood substrates. Also used for this purpose, for example, are pigment grade Bametaborate, known in the prior art, or Mg(OH).sub.2 which is recommended as a stain inhibitor by U.S. Pat. No. 4,218,516, issued Aug. 19, 1980. Also, U.S. Pat. No. 3,846,148, issued November 1974, discloses the chemical composition and manufacturing procedure of such products, comprising base pigments which essentially consist of wollastonite, talc or mica in combination with phosphate or borate of Ca or Zn and as a doping agent or active additive, one amphoteric metal hydrate of Al, Ti, Zr, Zn or Si. While the above-identified '516 Patent, refers to all of the specified metal hydrates as amphoteric, it is believed that Si hydrate is typically acidic, whereas Ti and Zr hydrates are considered compounds of basic character. Consequently, the tannin stain blocking activity of pigment compositions, as claimed, is not necessarily correlatable with the components' amphoteric character. In a related sense, it will be also observed (as documented in the literature) that totally or partially dehydrated metal hydrates (such as alumina, silica, zirconia, etc., essentially the above-specified doping agents) are characterized by variable and considerable degrees of (Bronsted and Lewis) surface acidity. Based on that, it is reasonable to suppose that no significant chemical reaction could occur between such substrates, characterized by surface acidity and dissolved, weakly acidic tannin species, and thus it is believed that the stain blocking capacity of composite pigments according to the above identified U.S. Patent results primarily from their barrier function and absorption capacity.
It can be concluded that stain inhibitors containing the specified metal hydrates as functionally active additives, according to the above identified U.S. patent, function essentially by reducing the permeability of coating systems and, thus display relatively limited tannin stain inhibitive capacity.